Bugfix: Improve polyfill function of log10 to return whole powers of …

…10 (chartjs#5275)

* Bugfix: Improve polyfill function of log10 to return whole powers of 10
        as integer values, as it caused endless loop in IE11 in the tick
        creation loop.

* Compare floating-point numbers directly instead of using unnecessary division.

src/core/core.helpers.js

@@ -159,7 +159,13 @@ module.exports = function(Chart) {
 			return Math.log10(x);
 		} :
 		function(x) {
-			return Math.log(x) / Math.LN10;
+			var exponent = Math.log(x) * Math.LOG10E; // Math.LOG10E = 1 / Math.LN10.
+			// Check for whole powers of 10,
+			// which due to floating point rounding error should be corrected.
+			var powerOf10 = Math.round(exponent);
+			var isPowerOf10 = x === Math.pow(10, powerOf10);
+
+			return isPowerOf10 ? powerOf10 : exponent;
 		};
 	helpers.toRadians = function(degrees) {
 		return degrees * (Math.PI / 180);

test/specs/core.helpers.tests.js

@@ -194,8 +194,12 @@ describe('Core helper tests', function() {
 
 	it('should do a log10 operation', function() {
 		expect(helpers.log10(0)).toBe(-Infinity);
-		expect(helpers.log10(1)).toBe(0);
-		expect(helpers.log10(1000)).toBeCloseTo(3, 1e-9);
+
+		// Check all allowed powers of 10, which should return integer values
+		var maxPowerOf10 = Math.floor(helpers.log10(Number.MAX_VALUE));
+		for (var i = 0; i < maxPowerOf10; i += 1) {
+			expect(helpers.log10(Math.pow(10, i))).toBe(i);
+		}
 	});
 
 	it('should correctly determine if two numbers are essentially equal', function() {